1. Technical Field
The present invention relates generally to semiconductor-on-insulator (SOI) devices and methods of making, and more specifically to SOI transistor devices having reduced floating body effects.
2. Description of the Related Art
Conventional or bulk semiconductor devices are formed in semiconductor material by implanting a well of either P-type or N-type conductivity silicon in a silicon substrate wafer of the opposite conductivity. Gates and source/drain diffusions are then manufactured using commonly known processes. These form devices known as metal-oxide-semiconductor (MOS) field effect transistors (FETs). When a given chip uses both P-type and N-type, it is known as a complimentary metal oxide semiconductor (CMOS). Each of these transistors must be electrically isolated from the others in order to avoid shorting the circuits. A relatively large amount of surface area is needed for the electrical isolation of the various transistors. This is undesirable for the current industry goals for size reduction. Additionally, junction capacitance between the source/drain and the bulk substrate and xe2x80x9coffxe2x80x9d state leakage from the drain to the source both increase power consumption. Junction capacitance also slows the speed at which a device using such transistors can operate. These problems result in difficulties in reducing the size, power consumption, and voltage of CMOS technology devices.
In order to deal with the junction capacitance and xe2x80x9coff statexe2x80x9d leakage problem as well as obtain reduced size, semiconductor-on-insulator technology (SOI) has been gaining popularity. A SOI wafer may be formed from a bulk silicon wafer by using conventional oxygen implantation techniques to create a buried oxide layer at a predetermined depth below the surface. The implanted oxygen oxidizes the silicon into insulating silicon dioxide in a gaussian distribution pattern centered at the predetermined depth to form the buried oxide layer. Field effect transistors formed on SOI substrates also may be able to achieve higher speed operation with higher drive currents, when compared with FETs formed on conventional bulk silicon substrates.
However, one problem with forming field effect transistors on an SOI wafer is the floating body effect. The floating body effect occurs because the buried oxide layer isolates the body of the transistor from the fixed potential silicon substrate and therefore the body takes on charge based on recent operation of the transistor. The floating body effect causes the current-to-voltage curve for the transistor to distort or kink, which in turn causes the threshold voltage for operating the transistor to fluctuate. This problem is particularly apparent for passgate devices such as those used in dynamic random access memory (DRAM) wherein it is critical that the threshold voltage remain fixed such that the transistor remains in the xe2x80x9coffxe2x80x9d position to prevent charge leakage from the storage capacitor.
Accordingly, there is a strong need in the art for a semiconductor circuit structure, and a method for forming such structure, that includes the low junction capacitance and low xe2x80x9coffxe2x80x9d state leakage characteristics of the SOI FET based circuits but does not suffer the disadvantages of a floating body potential.
A transistor on an SOI wafer has a subsurface recombination area within its body. The recombination area includes one or more doped subsurface islands, the doped islands having the same conductivity type as that of a source and a drain on opposite sides of the body, and having an opposite conductivity type from the remainder of the body. The doped subsurface island(s) may be formed by a doping implant into a surface semiconductor layer, for example through an open portion of a doping mask, the opening portion created for example by removal of a dummy gate. The doping of the islands may be performed so that the doping level of the island(s) is approximately the same as that of the body, thus enabling both Shockley-Read-Hall (SRH) and Auger recombination to take place.
According to an aspect of the invention, a semiconductor-on-insulator (SOI) transistor device includes an insulating layer made of an insulating material; an active layer of semiconductor material atop the insulating layer, the active layer including a body between a source and a drain, wherein the source and the drain are of a same conductivity type, and wherein the body is of an opposite conductivity type; and a gate atop the body. The body includes an island which is of the same conductivity type as the source and the drain, wherein the island is not in contact with the source or the drain.
According to another aspect of the invention, a semiconductor-on-insulator (SOI) transistor device includes an insulating layer made of an insulating material; an active layer of semiconductor material atop the insulating layer, the active layer including a body between a source and a drain, wherein the source and the drain are of a same conductivity type, and wherein the body is of an opposite conductivity type; and a gate atop the body. The body includes subsurface means for recombination due to both Auger recombination and Shockley-Read-Hall (SRH) recombination.
According to yet another aspect of the invention, a method of producing a semiconductor-on-insulator (SOI) transistor device includes the steps of forming a subsurface doped island in a surface semiconductor layer, the island having an opposite conductivity from that of semiconductor material surrounding the island; and forming a gate atop the surface semiconductor layer such that the island is at least partially underneath the gate.